Field of the Invention
The invention relates to an unloader for a valve element of a compressor valve, the unloader including a number of unloader fingers and a guide bushing. The invention also relates to a method for producing such a unloader.
The Prior Art
Unloaders are known in a variety of embodiments and are used for regulating plate and ring valves for compressors and the like. In principal unloaders are used for preventing the valve from closing and thus enables backflow of the medium to be compressed out of the compression space, generally the suction chamber. Continuously holding it open can stop the output of the compressor or cause the compressor to idle. Furthermore, lifting valve elements is frequently used for starting up the compressor with a minimal load or even to attain stepless control of the compressor.
Due to the high stress and the high dynamics, most unloaders are made from a solid piece. The turning, milling, and cutting operations cut away more than 80% of the starting material. The unloaders produced from a solid piece are stable and extremely vibration resistant; however, they are very expensive due to the high material and production costs.
DE15 03 426 A1, for instance, provides as a more cost-effective variant a unloader that comprises a metal bushing, concentric gripper rings, ribs arranged in a star shape, and unloader fingers that cooperate with a valve plate. The bushing is fabricated in the normal manner by turning or casting. The concentric gripper rings carry the metal unloader fingers, which cooperate with the valve plate, and are connected to one another by the ribs arranged in a star shape and to the bushing. The individual components are held together using weld joints. To provide accessibility during the joining process, only a small number of the ribs arranged in a star shape are embodied continuous from the outermost gripper rings to the bushing, in order to still provide adequate stiffness, the outer gripper rings are joined to one another by additional short ribs.
It is to be considered a drawback that a great number of welding processes, and significant time, are required for production. It should also be noted that the bushing must comprise a metal material that permits joining by welding. Based on this, with respect to slide properties, it must be assumed that the material pairing between bushing and the associated guide pins on the valve are not optimum. This drawback is particularly significant in dry running applications in which lubrication is not possible for various reasons. In addition, thermal deformation during the joining process can make it necessary to re-machine the bushing, especially the guide surfaces. Since all of the components in the unloader are made of metal, its weight is also therefore correspondingly heavy, which then limits the possible dynamics.
EP 0 888 770 B1 provides an unloader for compressor valves having a carrier and a number of unloader fingers secured thereto. At their end facing the carrier, the unloader fingers are provided with a retaining element that makes a positive fit with the carrier. This retaining element has a positioning element and a fixing element that snaps to the carrier in the assembly position. The center of the carrier is embodied as a bushing through which the carrier is guided on a guide sleeve. To improve the guidance and to reduce friction losses, slide rings are provided inside the guide sleeve.
The complex configuration of the guide region is a drawback with respect to ensuring appropriate positive slide conditions, even for dry running applications. The limited selection of materials for the unloader fingers, which materials must have both adequate strength for operation and appropriate elasticity for the snap-in connection, must be considered a drawback. Although compared to the variant described in the foregoing in which the unloader is fabricated from a solid piece, the materials expense is significantly reduced, due to the multi-part construction the loading of the unloader fingers can result in clearance, especially on the contact surfaces between the unloader fingers and the carrier. The weight is slightly reduced compared to the aforesaid DE15 03 426 A1; nevertheless, due to its weight the heavy metal carrier and the bushing formed thereon permit only limited dynamics.
One object of the present invention is to be able to manufacture, while reducing the weight and as cost-effectively as possible, an unloader of the aforesaid type for a valve element of a compressor valve, and to prevent signs of wear, such as for instance the occurrence of clearance between unloader fingers and their carrier structure.